4.5.3 Making Each Character a Separate Field

There are times when you may want to examine each character
of a record separately. This can be done in gawk by
simply assigning the null string ("") to FS. (c.e.)
In this case,
each individual character in the record becomes a separate field.
For example:

Traditionally, the behavior of FS equal to "" was not defined.
In this case, most versions of Unix awk simply treat the entire record
as only having one field.
(d.c.)
In compatibility mode
(see Options),
if FS is the null string, then gawk also
behaves this way.